Last August, when President George W. Bush banned the use of federal funds for any stem cell research that would require the creation and destruction of human embryos, one of the arguments his administration used was that embryonic stem cells might be unnecessary. Because such all-purpose cells could instead be isolated from adults and appeared to work in transplantation studies involving animals, administration officials alleged, why would anyone need stem cells derived from embryos, with their moral and ethical overtones?

The answer, possibly, is that what researchers once thought were stem cells from adults might not be. The premise of adult stem cell transplantation is that such cells are essentially undifferentiated and have therefore retained the capacity to become tissues as diverse as brain and liver. But two studies in the April 4 Nature suggest that transplanted adult stem cells merely fuse with a recipient's own cells without becoming a particular type of differentiated cell. If the results of the studies are upheld, it could bolster the case for using stem cells from embryos rather than adults.

This article was originally published with the title The Child Within.

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